Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, optical fiber system comprises a source of information-carrying optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the optical signals and demodulating the information they carry. Optical amplifiers are typically located along the line at regular intervals, and add/drop nodes are disposed at suitable locations for adding and dropping signal channels.
Variations in the temperature of an optical fiber can affect its transmission properties. Optical communication systems are usually based on high purity sitica optical fiber as the transmission medium. Variations in temperature change the group velocity dispersion (GVD) of silica fiber, subjecting different wavelength components to slightly different propagation time delays. Such dispersion can give rise to undesirable distortion of transmitted light pulses which, in turn, can limit bandwidth and/or transmission distances.
As the bit rate of optical fiber communications systems increases, the effect of temperature variations becomes increasingly significant. An optical transmission fiber is typically subjected to temperature variations on the order of 30-40 C. At present bit rates of 10 Gbit/s temperature variations have relatively little effect. But the effect of temperature variations on dispersion scales with the square of the bit rate. Thus in contemplated systems having four times the current bit rate (contemplated 40 Gbit/s systems), the effect of temperature variation increases by a factor of sixteen to become a formidable challenge to the operation of the system.
This challenge can be illustrated by a simple numerical example. While the magnitude of the temperature-induced variation in dispersion varies from fiber to fiber, a good estimate of the effect is 0.0025 ps/(nm.multidot.Km)/C (See W. H. Hatton et al., "Temperature dependence of chromatic dispersion in single mode fibers", J. of Lightwave Techn., Vol. LT-4, pp. 1552-55 (1986). Proposed 40 Gbit/s fiber communication systems involve fiber spans of 1000 Km which are subjected to temperature variation of 30 C. Such a system would be subjected to a temperature induced change in dispersion of about 60 ps/nm. This change would cause serious degradation of the 40 Gbit/s system. In a faster 100 Gbit/s system the impact of even a few degrees in temperature variation would be enormous.
A proposed solution to temperature variation was set forth by Kuhwahara et al. in "Adaptive Dispersion Equalization by Detecting Dispersion Fluctuations Using PM-AM Conversion", Electronic Letters, Vol. 34, p. 1956 (1998). Kuhwahara et al. recognized that temperature variation induced changes in dispersion would be critical, and they proposed that the dispersion be equalized by wavelength tuning of the signal wavelength. In other words, they proposed that the total dispersion for a given channel be maintained by adjusting the wavelength of the transmitter in response to dispersion variation.
The difficulty with the Kuhwahara proposed is that an optical fiber communication system contains many components that are highly wavelength dependent. Kuhwahara would require all these components, including add/drop filters and demultiplexers, to track unpredictable changes in the signal wavelength. This is unacceptable in a high speed WDM fiber communication system.
Accordingly there is a need for an optical fiber communication system which can compensate temperature induced changes in dispersion without changing basic network parameters.